Casting in a lost mold is a widely-used method for producing near-net-shape parts. Following casting the mold is destroyed and the casting is removed.
Molds are negative, they contain the cavity into which pouring takes place resulting in the casting to be produced. The internal contours of the future casting are formed by cores. In the production of the mold a model of the casting to be produced is used to form the cavity in the molding material. Internal contours are represented by cores which are formed in a separate core box. For lost molds and cores in the main refractory, granular materials are used as molding materials such as for example, washed, graded quartz sand. Other molding materials are for example zirconium sands, chromite sands, chamottes, olivine sands, feldspathic sands and andalusite sands. For production of the casting molds the molding materials are bonded with inorganic or organic binding agents. Bentonite or other clays are frequently used as inorganic binding agents. The molding materials are compacted in order to increase the strength. Often, in particular for the production of cores, hardening molding materials bonded with inorganic or organic synthetic resin binders are used. The curing takes place on the basis of a chemical reaction in a hot or cold process. Often such molding materials are also gas-flushed for the purpose of curing. The curing of the binding agent can also take place by heating of the molding material and expulsion of a solvent, which then brings about curing.
Normally the surfaces of the molds and cores are coated with a refractory coating. Ready-to-use refractory coatings (refractory coating agents) for coating molds and cores are suspensions of fine-grained, refractory to highly refractory inorganic materials in a carrier fluid, such as water or a solvent. The refractory coating is applied using a suitable application process, such as spraying, immersing, flooding or painting, to the inner contour of the casting mold or to the core and dries onto this so that a coating on the basis of a refractory coating (refractory coating film) results. The drying of the coating on the basis of the refractory coating can take place by addition of heat or radiated energy, e.g. by microwave radiation, or by drying in the ambient air. In the case of refractory coatings containing solvents the drying can also take place by burning off the solvent.
The coatings on the basis of a refractory coating should, inter alia, perform the following functions:    1. Improve the smoothness of the cast surface    2. Allow a clean separation of the liquid metal from the mold    3. Avoid chemical reactions between the molding material and the melt, thereby simplifying the separation of the molding material from the casting    4. Avoid surface defects on the casting, e.g. gas bubbles, penetrations, veining and scaling.
The abovementioned functions 1 through 3 are as a rule performed by combinations of various suitable refractory materials. Refractory here indicates materials and minerals which are able to withstand for a short time the temperature loading during casting of an iron smelt, and highly refractory applies to materials and minerals which are able to withstand for a short time the casting heat of a steel smelt. Examples of refractory materials used are mineral oxides such as corundum, magnesite, quartz, chromite and olivine, as well as silicates such as zirconium silicate, chamotte, andalusite, pyrophyllite, kaolinite, mica and other clay minerals individually or in combination. Graphite and coke are likewise used. The refractory materials are suspended in a carrier fluid. For the carrier fluid, solvents such as ethanol or isopropanol can be used, although these days water is in most cases the preferred carrier fluid.
Other base materials for refractory coatings are suspending agents such as for example clays that are swellable in water such as smectite, attapulgite or sepiolite or swellable organic thickeners such as for example cellulose derivates or polysaccharide. A refractory coating also contains a binding agent, in order to fix the refractory materials to the molding material. As a rule here synthetic resins or synthetic resin dispersions are used such as for example polyvinyl alcohol, polyacrylate, polyvinyl acetate and corresponding copolymers. Natural resins, dextrins, starches and peptides can also be used as binding agents. The abovementioned swellable clays can likewise perform the functions of the binding agent.
Refractory coatings can contain further additives, in the case of aqueous refractory coatings in particular preservatives and rheologically-acting additives and floating agents. Rheologically acting additives and/or floating agents are used in order to set the desired flow properties of the refractory coating for processing. In the case of aqueous refractory coatings wetting agents can also be used in order to achieve a better wetting of the molding material. A person skilled in the art will be aware of ionic and non-ionic wetting agents. By way of example as an ionic wetting agent dioctyl sulfosuccinates and as a non-ionic wetting agent alkine diols or ethoxylated alkine diols are used
Because of the complexity of today's castings the function of coatings on the basis of a refractory coating of avoiding surface defects on the casting is in particular becoming important. Because core geometries are becoming increasingly filigree and the molds ever-more complex, increasing the demands on the molding materials and in particular the refractory coatings. As a result of the thermal expansion of the sand contained in the molding material due to the casting heat inorganic and in particular synthetic resin-bonded molds and cores can rip open so that the liquid metal penetrates the mold or the core. The resultant surface defects, such as veining, can only be removed with difficulty.
During the pyrolysis of synthetic resin-bonded molding materials gases are generated by the casting heat. These can lead to casting defects. In this connection, various causes can be identified that lead to these casting defects which are known as gas defects.
On the one hand gas defects as described by H. G. Levelink, F. P. M. A. Julien and H. C. J. de Man in Gieβerei 67 (1980) 109, can be caused by “exogenous gases”. These “exogenous gases” mainly result during the pyrolysis of organic binding agents upon contact with the metal smelt in the mold or the core. These gases create a gas pressure in the molding material, which, if it exceeds the metallostatic counterpressure, can lead to gas defects in the casting, in most cases in the upper area thereof. These gas bubbles as a rule have a smooth inner surface.
A further kind of gas defects is described for example by Gy. Nandori and J. Pal. Miskoloc along with K. Peukert in Gieβerei 83 (1996) 16. Here it is a case of gas bubbles which occur accompanied by slaggy patches. The causes of such gas-slag defects can be seen as exogenous, i.e gases resulting from the molding material and mold cavity, and “endogenous”, i.e gases resulting from the smelt. These gases react to some extent with the smelt resulting in oxide-rich slag. Together with the remaining gases this slag causes gas defects. An influencing factor in the formation of these gas defects is the gas permeability of the molding material covered with the coating on the basis of a refractory coating.
At points where the surface of a core or a mold is not adequately protected against the infiltration of smelt, penetrations frequently occur. These defects have to be removed from the casting at great effort.
During the casting process the coating on the basis of a refractory coating can scale off from the core or the mold, if within the core a high gas pressure develops as a result of pyrolysis of the molding material binder and the refractory coating because of a low gas permeability offers a high resistance to this pressure. If the gas pressure here exceeds the adhesion of the coating on the basis of a refractory coating on the core or the mold, then the refractory coatings will scale off. Casting defects as a result of ascending refractory coating particles in the smelt are the result.
Attempts have previously been made to develop refractory coatings which counteract these casting defects. For example, by the addition of platelet-shaped layer silicates such as for example calcinated kaolin, pyrophyllite, talcum and mica or other clay minerals to the refractory coating, coatings on the basis of a refractory coating on the molds or cores result which under the effects of tensile forces deform easily. The individual platelets overlap with one another and are thus able to easily cover cracks which as a result of the thermal expansion of the sand occur in the molding material. Because of their dense texture coatings on the basis of a refractory coating, containing platelet-shaped layer silicates, have only low gas-permeability however. Gases generated during the thermal decomposition of the binding agent of the molding material can then only pass through these layers with difficulty and high gas pressures develop which can lead to the abovementioned gas defects and scaling defects.
Patent application WO 2007/025769 describes refractory coatings (indicated there, together with molding material mixtures, also as molding compounds), containing a borosilicate glass additive in a proportion of at least 0.001%, preferably at least 0.005 wt. %, in particular at least 0.01 wt. % in relation to the solid matter content of the refractory coating. The proportion of borosilicate glass is preferably selected to be less than 5 wt. %, in particular preferably less than 2 wt. % and quite particularly preferably within a range of 0.01 through 1 wt. %, in each case in relation to the solid matter content of the refractory coating. According to a particularly preferred embodiment borosilicate glass in the form of hollow microspheres, that is to say small hollow balls with a diameter of the order of preferably 5 through 500 μm, particularly preferably 10 through 250 μm, the shell of which is made of borosilicate glass, is used. It is assumed that the borosilicate glass under the effect of the temperature of the liquid metal melts and as a result cavities are released which can compensate for the volume expansion of the casting material caused by the casting heat. The softening point of the borosilicate glass is preferably set in the range of less than 1500° C., in particular preferably in the range 500 through 1000° C. If these refractory coatings are used flaking of the coating on the basis of a refractory coating under the influence of the liquid metal occurs only extremely rarely. In addition, it has been found that no veining occurs so that a smooth casting surface is obtained.
Since according to WO2007/025769 melting of the hollow balls of borosilicate glass is intended, after the melting of the hollow balls the coating on the basis of a refractory coating has holes through which the liquid metal can penetrate the surface of the core or the mold. As a result there is a danger of penetration defects. This problem also cannot be solved by using borosilicate glass balls with a higher melting point, for in addition to the network former boroxide glasses also contain so-called network modifiers such as sodium oxide and potassium oxide, wherein all three compounds with practically all the above mentioned constituents of refractory coatings (apart from carbon or graphite), in particular all platelet-shaped clay minerals and silicates, form low-melting compounds. In addition hollow balls in borosilicate glass have only low mechanical stability. Therefore they rupture very easily under compressive loading, which in the production of refractory coatings is unavoidable. A further disadvantage in the use of hollow balls of borosilicate glass is their strong alkalinity. This leads to an unfavorable change in the pH value of the refractory coatings. Therefore according to a variant of the molding compound of WO2007/025769 the addition of an acid or acid source is provided for.
From WO 94/26440 refractory coatings are known which in relation to the weight of the ready-to-use refractory coatings have a content of inorganic hollow balls of 1 through 40%, preferably of at least 4%, or even at least 10%. The hollow balls consist of for example silicates in particular of aluminum, calcium, magnesium and/or zirconium, oxides such as aluminum oxide, quartz, magnesite, mullite, chromite, zirconium oxide and/or titanium oxide, borides, carbides and nitrides such as silicon carbide, titanium carbide, titanium boride, boron nitride and/or boron carbide, or carbon. But hollow balls of metal or glass can also be used. These hollow balls are effective in numerous ways. Thus the dense packing of the base material particles in the refractory coatings, which can be seen as the main cause of the low gas permeability, is relaxed by the small balls and rendered more gas permeable. It is also assumed that at the start of the casting process the insulating properties of the hollow balls and the gas-permeable coatings on the basis of a refractory coating cause a delay in heat transfer through the refractory coating into the molding material. Subsequently the hollow balls melt under the casting heat and/or rupture under the casting pressure, whereby in the coating on the basis of a refractory coating numerous micro-flaws result so that the gas permeability of the coating on the basis of a refractory coating is increased.
Here also, because of the large quantity of melting hollow balls there is the possibility that with an unfavorable overlapping of individual hollow balls in the coating on the basis of a refractory coating holes may form meaning that the casting may have penetration defects.